In general, rails for use in railroads are formed through heating the billet and hot-rolling it into a specific form, and then, after performing heat treatment according to the desired mechanical properties, it is cooled to ambient temperature. Then, after performing rectification, a specific examination can be performed and the rail becomes a final product. Heat treatment is performed as necessary, and there are instances where these operations may be omitted.
In the above-described rail manufacturing method, it is normal to perform the hot-rolling process while the rail is positioned laterally. When no heat treatment is performed, the rail is transported on its side to the cooling bed, where it is cooled.
However, as the cross-sectional shape of the rail is asymmetrical in the vertical direction when it is in the upright state, curvature can be generated in the height direction during the cooling process after hot-rolling (here, we refer to curvature in the vertical direction when the rail is upright as being bending in the height direction, and curvature in the lateral direction as being bending in the width direction). In normal operational methods, as the bending in the height direction may increase and it is easy for the rail to become unbalanced and topple over, this causes difficulties in the normal transport of the rail, in the placing of the rail on the cooling bed, and in the withdrawal of the rail from that bed. Therefore, from the view of trying to prevent this unbalanced state, in most of the above manufacturing processes, the rail is treated and transported on its side. However, when rapidly cooling the rail using air or mist, this cooling operation is performed on the rail when it is upright, but, as described in Japanese Unexamined Patent Application Publication S62-13528, it is common for the heat treatment to be performed on the rail in an upright state, and then, the rail is positioned laterally until it reaches the cooling bed.
When leaving the rail on its side and letting it cool in this manner (i.e., by allowing the heat to naturally dissipate without forcible cooling), it becomes easier for the rail to bend, as there are no constraints on the rail in the height direction. Further, as a temperature difference develops between the side surface of the rail which is closest to the cooling bed and the opposite side surface, bending can also occur in the width direction.
This type of rail curvature is rectified at the end of the manufacturing process whereby rails which have developed curvature being placed on a rectifier that has rollers arranged in a zig-zag shape, and undergoing a further press operation as necessary. However, as this rectification process can require a great deal of time if the amount of curvature is large, it can result in a reduction in productivity or an increase in manufacturing costs. Further, for rails to be used in the high-speed railroads which have been in demand recently, as these rails demand an especially high level of straightness, instances may arise where it is not possible to sufficiently rectify the curvature by press rectification, leading to a reduction in yield.
As methods of controlling curvature on the cooling bed, the following types of technology have been disclosed.
First, in Japanese Unexamined Patent Application Publication H05-076921, a method is described in which the high temperature rail is cooled on its side on the cooling bed, and both ends of the rail which is charged within the cooling bed are bent such that the head of the rail moves to the outer side of the bend. Further, in Japanese Unexamined Patent Application Publication H09-168814, a method is described in which a transfer and a stopper are used on the cooling bed to bend the lateral rail such that it will be straight after cooling.
However, in these methods, it may be difficult to adjust the degree of curvature and the shape of this curvature of both ends of the rail and, and it is not possible to rigorously control this curvature. Further, it may be difficult to control the curvature in the width direction of the rail.
In Japanese Unexamined Patent Application Publication S59-031824, a method is described in which curvature of the rail during the cooling process is prevented by setting the rail in an upright state, insulating the bottom part of the rail, and synchronizing the cooling speed of the foot of the rail with the cooling speed of the head of the rail. By this method, the curvature of the rail is reduced, but it is difficult to select insulation in order to synchronize the cooling speeds of the foot and head of the rail, and capital investments may increase. Further, the time required for cooling will likely grow due to this insulation in order to decrease the cooling speed, resulting in a decrease in productivity.
In addition, when performing the above type of insulation on multiple rails, if the cooling conditions for all of the rails are the same, then there is efficacy in straightening the rails, but if rails of differing sizes are mixed together in the cooling process, the cooling conditions for each rail may differ, resulting in rails for which the curvature is not reduced. Further, but as the time required for the cooling process will grow, ample time is allowed for expansion and contraction of the material to occur, leading to concerns that the amount of curvature may actually be increased.